Hey Mitch, I found the following page...which is a class lesson on humidity and dew point.
http://www.faqs.org/faqs/meteorology/temp-dewpoint/
There are a bunch of equations, so some of it is a bit over the top, but most interesting is this from the introduction section:
If you have a closed container with water and air (like a beaker) then there an equilibrium will develop, where the air will contain as much vapor as it can.
So the closed container is going to affect the experiment. It should reach an equilibrium, but it will be dependent upon the amount of total water in there, as well as the temperature and the pressure. Pressure at different altitudes will affect the RH for a given temperature. Both pressure and temperature will affect the density of air, which can affect the vapor pressure. If I understand things correctly.